CODEWITHSUNDEEP
java
gdfbarbosa
gdfbarbosa

Reputation: 835

Unpredictability of the BigDecimal(double) constructor

I started using Sonar recently in a project, and i got a PMD rule broken about using the constructor new BigDecimal(double val). When i read the java documentation, i found that new BigDecimal(double val) is somewhat unpredictable and that I should use new BigDecimal(String val) which is predictable.

Here is what javadoc says for BigDecimal public BigDecimal(double val):

Translates a double into a BigDecimal which is the exact decimal representation of the double's binary floating-point value. The scale of the returned BigDecimal is the smallest value such that (10scale × val) is an integer.

Notes:

The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.

The String constructor, on the other hand, is perfectly predictable: writing new BigDecimal("0.1") creates a BigDecimal which is exactly equal to 0.1, as one would expect. Therefore, it is generally recommended that the String constructor be used in preference to this one.

When a double must be used as a source for a BigDecimal, note that this constructor provides an exact conversion; it does not give the same result as converting the double to a String using the Double.toString(double) method and then using the BigDecimal(String) constructor. To get that result, use the static valueOf(double) method.

Why does this constructor really exists? Isnt new BigDecimal(String val) enough for that matter? When should I use the new BigDecimal(double val) constructor?

Upvotes: 25

Views: 12054

Answers (6)

Angel O'Sphere
Angel O'Sphere

Reputation: 2666

double val = 0.1; // this is not 0.1, but an approximination as pointed out in other answers.
BigDecimal biggy = new BigDecimal(val); // this variable conatains the "same" value as val does, however it is not 0.1, but as pointed out in other answers: 0.100000000000000005551115123`

So, in the case listed here, where you have full control over the source code, you should write the val as 'String val = "0.1"'

However the contructor BigDecimal(double val) is usefull in case you read binary doubles from a file. Obviously you will want in nearly all cases BigDecimals that are exactly the same as the 'doubles'

Upvotes: 0

Martin Andersson
Martin Andersson

Reputation: 19583

Unpredictable? I disagree.

final double before = 0.1;
BigDecimal bd = new BigDecimal(before);
double after = bd.doubleValue();
assert before == after;

Key to understanding:

assert 0.1 == 0.1000000000000000055511151231257827021181583404541015625;

If you truly meant "0.1", then yes, use the String constructor =) But if you have a double already, then that double is not "0.1".

The JavaDoc you quoted says:

When a double must be used as a source for a BigDecimal, note that this constructor provides an exact conversion;

I fail to see how an exact conversion is "unpredictable" and I am very much in favor of removing the word "must" from this quote.

At the end of the day, both constructors work just like intended. If you have String "0.1", then BigDecimal(String) convert this value exactly. If you have a double 0.1, then BigDecimal(double) convert this value exactly.

Upvotes: 1

Peter Lawrey
Peter Lawrey

Reputation: 533750

Why does this constructor really exists?

It converts the actual represented value of double to a BigDecimal. The whole point of BigDecimal is to give as much precision as possible and that is what this constructor does.

If you want to take the value you would get with a small amount of rounding the Double.toString(double) uses you can use

System.out.println(BigDecimal.valueOf(0.1));

prints

0.1

When should I use the new BigDecimal(double val) constructor

When you want to know the value double really represents. You can apply your own rounding as required.

When you use double you should always apply a sensible rounding. But, if you did that you may find you don't need BigDecimal. ;)

Upvotes: 18

dantuch
dantuch

Reputation: 9293

When should I use the new BigDecimal(double val) constructor?

Preferably - nowhere.

If you are willing to play with BigDecimals, using constructor with double means loosing all benefits of exact number presentation.

Why does this constructor really exists?

Because sometimes you have only double, that you want to translate to BigDecimal. Forcing you to use .toString() inbetween would be just silly ;)

Upvotes: 7

Philipp
Philipp

Reputation: 69703

You would use the double constructor when the data you want to work with already exists as a double value. Having to convert it to a string before converting it to BigDecimal would be a wasteful in that case.

Upvotes: 1

SJuan76
SJuan76

Reputation: 24895

Because if you already begin with a double value as data, you have already lost that precission. So not having it would force you to convert it to String for BigDecimal to convert it back.

And also, maybe sometimes you just need that value.

Upvotes: 4

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